The detention basin being constructed at Garnett Lehmann Park is one of a series of projects under the Gowrie Creek Catchment Flood Mitigation Project to substantially increase flood protection for Toowoomba. The detention basin, in conjunction with the one under construction at Ballin Drive Park and other works along East Creek, follows extensive flood studies.
Substantial progress has been made on the Garnet Lehmann Park Detention Basin with the project nearing completion.
Earthworks to create the embankments for the 67ML detention basin are nearing completion while installation of the twin 1500mm x 1200mm box culverts has been completed.
The maximum height of the bund wall (the embankment) above the floor of the basin is 4.8 metres, with the height much less away from the bank of the creek. The height of the embankment above the culvert/channel invert is only six metres.
A section of East Creek in the basin area has been reshaped, fish lunkers have been installed under the creek banks to encourage breeding and rock works in the creek have also been completed. Other works completed include stormwater pipe alterations and sewer relining.
Landscaping of the detention basin is under way. Plantings include a mix of water plants, mature trees, shade trees, wildlife-attracting shrubs and flood-tolerant vegetation to improve bio-diversity. There will also be a riparian area with high-order environmental values. While 289 mature trees were removed, the new plantings include 182 new large trees, 926 new small pot and tube stock trees, about 7300 new shrubs and about 21,000 new ground covers.
Council is continuing to take measures to minimise sediment run-off, dust and noise impacts during construction.
The project is being funded through the Natural Disaster Relief and Recovery Arrangements (NDDRA), which includes a joint State and Commonwealth contribution of $20 million with Council contributing $5 million.
- Release draft plan (see below). Completed November 2013.
- Public consultation and feedback gathering. Completed April 2014.
- Finalise plans. Completed July 2014.
- Commence construction, Scheduled to commence in August or September 2014.
- Landscaping of detention basins. Scheduled for late 2014 or early 2015.
- Offset replanting in East Creek corridor. Scheduled for 2015.
- Project completion is expected in late February/early March 2015, weather permitting.
A 67ML detention basin will be built in Garnet Lehmann Park and will include:
- landscaped design including a mix of water plants, mature trees, shade trees, wildlife-attracting shrubs and flood-tolerant vegetation
- new formal and informal paths through the basin
- new boardwalk crossing
- channel with natural appearance and planting along creek edge
- new shared pathway for pedestrians and cyclists
- new nesting boxes as well as retention of frog habitat and mulberry tree
- relocation of tree ferns.
Why Garnet Lehmann Park?
Various combinations of detention basins and various locations, including South and Long streets, were considered, but studies showed the best flood mitigation would be achieved with detention basins at both Garnett Lehmann Park and Ballin Drive Park.
The basins are designed to maximise flood mitigation to protect property, infrastructure and potentially save lives.
Community feedback is important and has been collected since November 2013. Residents have been informed of this project through the media and community information sessions and displays held between November 2013 to April 2014, as well as in Council's monthly newsletter, social media, website and video news updates.
The feedback gathered will be incorporated where possible in the final design.
Trees, wildlife & amenity
A significant number of trees will be removed to construct the detention basins. However, the design team is working to minimise this number, and Council has committed to replanting a minimum of two trees for each one removed as part of the project.
Council engaged independent experts, Red Leaf Projects, to evaluate the existing wildlife on the site and any possible impacts from construction. Red Leaf Projects were also asked to provide recommendations about minimising impacts during construction works.
The replanting will be a mix of mature trees, shade tree species and shrubs to attract birds and other wildlife. Trees that cannot be replanted in the immediate basin area will be replanted in nearby catchment locations.
The detention basin will have an earth embankment and the low-flow channel will be maintained. The area will be landscaped and it will not be bare earth. The creek will not be concreted over. There will be a small concrete area for sediment clean-out.
Council's Parks and Gardens Branch staff, including an arborist and landscape architects, are playing a key role in the design process. The finished product will be a landscaped park that can be used for the usual recreational activities, including off-leash dog walking, while providing a detention basin for flood mitigation.
The detention basins need to located upstream in East Creek to achieve the maximum flood mitigation benefit. They are being designed to handle a one in 100-year Annual Recurrence Interval rain event. The 2011 flood is considered to be a one in 500-year event.
Detention basins temporarily hold water during storm events and restrict the speed at which water continues moving down stream. They reduce flooding by allowing water to be collected and released in a controlled manner. The existing basins along West Creek reduced peak flow by about 30% in the 2011 flood. A similar result is expected for the East Creek detention basins in a one in 100-year rain event.
The basins have been proposed since the 1998 adoption of the Gowrie Creek Catchment Management Study. These basins were reviewed again in the 2007 Gowrie Creek flood risk and mapping study which revised modelling based on increased catchment conditions. This was then updated post-2011 flood to reflect best-modelling practice in a study undertaken by Aecom. Council has also undertaken internal studies on options analysis for basins.
Best use of resources
Following the 2011 floods, the Queensland Reconstruction Authority provided $25 million for flood mitigation projects in Toowoomba. Council approved construction of detention basins in Garnett Lehmann Park and Ballin Drive Park as part of the overall program.
Presentation made during Feb/Mar community engagement
View the presenation made during community engagement sessions and workshops on Slideshare.www.slideshare.net/ToowoombaRC/gowrie-creek-flood-mitigation-presentation-26-february-2014 >>